The actual control circuit within the starter can use any voltage, within reason, it is therefore necessary to consider various factors in determining the voltage to use.
The higher the voltage the more the danger from electric shock, however, the lower the voltage the greater the problem of volt drop in the control circuits.
The length of the circuits can vary considerably even from the same MCC therefore for consistency and commonality of components the voltage will need to be high enough to prevent volt drop becoming a problem.
There are standard voltages for relay and contactor coils, timers and ancillary components used throughout the industry it is important to select from those standard voltages, the two that are most widespread are 24V and 110V both ac and dc.
The use of dc will limit the selection of push buttons and indicators as most are designed for ac use, it will also limit the use of other ancillary components for the same reason, whereas ac has a far greater range of push buttons, indicators and ancillary components designed for its use.
Therefore ac is selected for the control circuits.
However the use of ac brings a problem due to cable capacitance which can cause relays and contactors to fail to de-energise when the circuit is de-energised, the higher the control voltage the greater the problem. The maximum permissible cable length that may be used in a control circuit decreases with the inverse of the voltage ratio squared; thus a four fold increase in voltage will decrease the maximum cable length by a factor of sixteen.
Due to the volt drop incurred the use of 24V is discarded as the maximum cable lengths would be reduced to 21% of the length if 110V were used.
The control voltage selection is therefore a compromise, 110Vac is used as it is the best compromise if the control relays and pilot contactors are limited to 8VA/2.6W sealed and 70VA/50W inrush which equates to 73mA sealed and 636mA inrush respectively.
For one relay sealed plus one relay inrush plus a time delay relay with a load of 5VA 46mA normal condition then the total load is 755mA as max load for 10% inrush volt drop and 191mA as max load for the sealed 2.5% drop.
From the loadings given in the paragraph above to limit the inrush volt drop to 10% the maximum length of 1.5mm² control cable is 469 metres, to limit the sealed volt drop to 2.5% the maximum length of 1.5mm² control cable is 466metres and to ensure relays do not self maintain on circuit de-energisation the maximum length of the control cable assuming three wire control is 422 metres.
Thus the maximum control system total cable length for the control system is 422 metres. It should be noted that increasing the cable size to minimise the volt drop will have little effect on the capacitive coupling effect, thus there is a 422 metres limit on the control system total cable length for cables sized at 1.5mm² or greater when the voltage is 110Vac.
The alternating current frequency throughout the world is either 50Hz or 60Hz to enable maximum utilisation of the design, coils and ancillary components suitable for use on both 50Hz and 60Hz should be selected.
Where the standard supply voltage is 400V three phase the control voltage should be derived from a single phase 400/110V transformer within the MCC which has it’s primary supply from the MCC. The transformer secondary side, that is the 110V winding, should have its neutral leg earthed.
There are other systems within the starter that are classed on the starter wiring diagrams as auxiliary circuits, these give voltage free signals to a relay panel or PLC which would be located locally to the starter and voltage free signals to the first control panel indication circuits for remote indication. Although the circuits are isolated by the starter fused switch auxiliary contacts when the starter is switched off, for additional safety these circuits are designed for use at 24Vdc.
Since each control panel has interface relays to drive the next control panel’s indication circuits the total circuit lengths are much reduced when compared to the starter control circuits. By selecting led lamps for the indicator units and push button indicator units where the load of each led is 17mA the indication circuit relays have minimal contact loading which permits the use of miniature relays that have a coil inrush and consumption of 125mA cold, 108.4mA warm. If, for example, the limit of volt drop is 2.5% then for four relays energised the maximum length between starter and CP1 and then each subsequent control panel is 79 metres.